For electrooptical display elements, the properties of nematic or nematic-cholesteric liquid crystal materials, which significantly change their optical properties, such as light absorption, light scattering, double refraction, reflecting power or color, under the influence of electric fields, are utilized to an increasing extent. The functioning of such display elements is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in a twisted cell or the cholesteric-nematic phase transition.
Industrial application of these effects in electronic components necessitates liquid crystal dielectrics which must fulfill a large number of requirements. Chemical stability towards moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges and constant and alternating electrical fields, is particularly important in this context. A liquid crystal mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C., and as low as possible a viscosity at room temperature, which should preferably be not more than 70.10.sup.-3 Pa.s, are also required of liquid crystal dielectrics which can be used industrially. Finally, these dielectrics must exhibit no characteristic absorption in the range of visible light, i.e., they must be colorless.
A number of liquid crystal compounds which fulfill the stability requirements demanded of dielectrics for electronic components and which are also colorless are already known. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In both these classes of compounds, and also in other known series of compounds having a liquid crystal mesophase, there are no individual compounds which form a liquid crystal nematic mesophase in the required temperature range of 10.degree. C. to 60.degree. C. Mixtures of two or more compounds are, thus, as a rule prepared in order to obtain substances which can be used as liquid crystal dielectrics. These are usually obtained by mixing at least one compound having a low melting point and clear point with another compound having a significantly higher melting point and clear point.
A mixture which has a melting point below that of the lower-melting component, while the clear point is between the clear points of the components, is usually obtained by this procedure. However, optimum dielectrics cannot be prepared in this manner, since the components with the high melting points and clear points almost always also impart a high viscosity to the mixtures. The switching times of the electrooptical display elements produced with these mixtures are thus undesirably lengthened.
It has thus been sought to prepare liquid crystal dielectrics which have a nematic phase in the required temperature range and which provide sufficiently short switching times in liquid crystal cells at room temperature.
Hexahydroterphenyl derivatives satisfying these requirements can be achieved for the range of liquid crystal dielectrics having positive dielectric anisotropy as disclosed in German Offenlegungsschrift No. 2,701,591 or its equivalent U.S. Pat. No. 4,154,697. However, these hexahydroterphenyl derivatives cannot be used to prepare liquid crystal dielectrics having negative dielectric anisotropy, which, for example, are required for liquid crystal display elements which operate by the principle of dynamic scattering.